Strength training doesn't increase size or weight. Eating does that. I usually lose weight when concentrating on strength work - it burns a lot of calories. I'm always trying to get more cut. And of course as with any strength work, the last rep of the last set, no matter the rep count, has to be the last because one is pretty sure the next rep would lack form. I thought it helped a lot from 100k on up. Back then, my 1RM squat was about my weight*1.7, done powerlifting style. My usual squat is ATG. Flexibility is good and I like the bounce at the bottom.

When I was restarting serious cycling, 30 years ago, Bicycling Magazine sponsored a 4-person team of over-50 riders to break a RAAM record, which they did. One of those riders could do 50 reps at 450 on the leg sled. Of course leg sled reps are pretty easy compared to squats. Cycling is just very high rep strength work which gives it a strong aerobic component. The idea is to condition the muscles until distance no longer matters. One should be just as fast after 400k as one was at the start, in terms of the aerobic component, i.e. HR and power about the same. That's totally trainable. Been there, done that. A good, i.e. sadistic, RBA will put some ~18% climbs near the end of a long brevet.

For whatever reason, but probably all the weight work, I would win all the hill sprints and most of the flat sprints with my cycling group, almost all of whom were 10-15 years younger than I. We, those of us who are left, are just ordinary cyclists, not racer boys, just folks who enjoy riding a lot. My BMI was 23-24 (still is). It took a lot more time to develop my climbing than it did my sprinting - aerobic ability takes at least 7 years of training to fully develop.