Just two weeks ago, it appeared that picking the winner of the Kentucky Derby would be an easy task. Timely Writer was the class of the field.

When he became ill, the task seemed easier still. Linkage towered over the remaining horses.

But when Linkage's trainer perversely chose to stay away from Churchill Downs, the 108th Derby was rendered almost inscrutable. None of the 20 horses who were entered this morning has the quality, the stamina and the acceleration to insure that he can surmount all the problems that inevitably will arise in such a large field. Luck will play an important role Saturday.

If the absence of a genuinely top-class horse weren't confusing enough, the entrants in this Derby have such far-flung origins that comparing them is very difficult. None of the favorites have raced against each other. Four horses in the Derby are coming out of the Wood Memorial Stakes at Aqueduct; three are coming from the Santa Anita Derby; three from the Arkansas Derby; three from the Derby Trial here. Before trying to figure out which horse will win Saturday, a handicapper might first ask: Where will he come from?

Wood Memorial: Frank LaBoccetta, trainer of the undefeated Air Forbes Won, maintains that his colt's slow winning time in the Wood was an aberration. It wasn't. So many horses of established mediocrity finished close to Air Forbes Won that the final times confirm that his performance was an undistinguished one.

Besides, Air Forbes Won has had the wrong kind of preparation to succeed in the Derby. He has crammed his entire career into a six-week period, while history suggests that a horse needs much more of a foundation to go 1 1/4 miles successfully. In 107 years, the Derby never has been won by a horse who did not race as a 2-year-old.

Throw out Air Forbes Won and all the horses who were behind him in the Wood: Wolfie's Rascal, Reinvested and Laser Light.

Santa Anita Derby: "I think we had about as good a crop of 3-year-olds on the West Coast this winter as we've ever had," trainer Ron McAnally maintained. Other Californians share his assessment. But the evidence doesn't support them.

The times of the major stakes this winter were not particularly fast, by California standards, and the performances of 3-year-olds who have ventured away from Santa Anita have not suggested that the group was anything special. Gato del Sol, who finished three lengths behind Muttering in the Santa Anita Derby, didn't make much of an impression when he plodded along to finish a distant second behind Linkage in the Blue Grass Stakes. Cassaleria, who lost the Santa Anita Derby by five lengths, went north and was beaten by a couple nonentities in the California Derby.

Muttering is the only California horse here who deserves respect, because trainer Wayne Lukas has demonstrated that giving a horse a layoff of a few weeks may prepare him to run an explosive race. But even if he is touched by Lukas' magic, Muttering may not be good enough. The rest of the California contingent--Cassaleria, Gato Del Sol, Water Bank and Rockwall--certainly are not.

Derby Trial: Both Royal Roberto and Star Gallant spent the winter in Florida and tuned up for the Derby here last Saturday, but the similarities between them end there. Star Gallant tired badly in the stretch and finished second behind a longshot; he looked as if jockey Bill Shoemaker will need to apply a respirator in the stretch run of the Derby. Royal Roberto, by contrast, closed well in the Trial, confirming the impression that he is a natural distance runner. He is a legitimate contender.

Arkansas Derby: No horse ever has run in the Arkansas Derby and gone on to win the Kentucky Derby. Nevertheless, there is evidence that the strongest contingent of 3-year-olds in the country was at Oaklawn Park.

Hostage defeated El Baba by running 1 1/8 miles in 1:51 2/5, a time that looks very slow. In fact, the track was extraordinarily slow. On the same afternoon, older allowance horses ran a route race in a time that was the equivalent of 1:53 for a mile and one-eighth. A 3-year-old named Listcapade ran in the equivalent of 1:53 in another allowance race that day, then came here and upset Star Gallant and Royal Roberto in the Derby Trial. Lesser 3-year-olds who were based at Oaklawn this winter have subsequently been running well against tough competition in New York.

Hostage always had looked like a horse of special quality, though an injury this week ended his career and denied him the opportunity to prove it. El Baba, who finished second while spotting the winner nine pounds, is obviously a very good horse. His loss in the Arkansas Derby was only the second in his 10-race career; the only other horrse to beat him was Linkage. The third-place finisher at Oaklawn, Bold Style, doesn't have a record that looks like that of a Derby horse, but with a great trainer like Jack Van Berg he merits respect.

Bold Style might be worth a flyer at a long price, but El Baba and Royal Roberto appear to be the two solid horses in this field.

By the end of the year, Royal Roberto may establish that he was the best horse in the Derby field. He looks like a natural for the Belmont Stakes. But he is the wrong kind of horse for this Kentucky Derby.

It is always tough to for a slow-starting stretch-runner to win in a large field, though horses like Pleasant Colony or Timely Writer could do it because they could accelerate so fast once they were in gear. Royal Roberto doesn't have this ability. He is a long-striding plodder, a big, unmaneuverable animal who seems ill-equipped to wend his way through a 20-horse field.

If the morning line is right, and he goes to the post at 20 to 1, I might find the price irresistible. But I would guess that he will get into a lot of trouble Saturday; I look forward to betting him with more confidence in a different type of race.

El Baba, by contrast, figures to get a more beneficial trip. He has the kind of controllable speed that usually enables him to make good racing luck for himself; his tractability even enabled him to beat Linkage once this winter.

Breaking from the No. 4 post position in a field where there isn't much early speed, he should be able to sit within striking distance of Star Gallant, wait for the leader to collapse, and move to the lead while the stretch-runners in the field are getting in each other's way. He is no mortal lock; he is not even a good betting proposition at odds of 5 to 2 or thereabouts. But he appears to be the right horse for this Kentucky Derby.