l took a break from climbing to thru-hike the AT, and lost a lot of upper body strength including in the fingers/tendons.I will gain the muscle back quicker than the fingers, so how can l train (whether on rock/plastic or otherwise) to get back to where l was pre-hike without popping a tendon in the process?

The biggest thing is to ease back into it. I've dealt with a lot of finger injuries, which were usually the result of not letting my tendons catch up with my muscles. It is hard to do sometimes, because u get psyched to be climbing again & want to try hard stuff, but ur tendons cant handle it & then u r out a month or two.

The best thing u can do if u r worried about ur fingers is simply stay off of small holds. Pinches, slopers & other such holds can get ur hands strong without straining ur tendons like crimps will. If u need to add more difficulty, just get on some steeper stuff.

Once u feel like you've built a good base after a couple of months, then I'd start training again on the small stuff. Hope that helps!

BJ Sbarra / SplitterChoss.com
http://splitterchoss.com/blog

l was in the same position as u (except l had exams, not a hike) - climbed loads, stopped for a while, then started again. The problem was l restarted at the grade that l was used to, & even trying to push up a grade higher.
The result of this was very sore finger joints. I've not seen a doctor so l do not for sure whether it was just a strain/sprain/stretch of the tendons as opposed to triggering a degenerative disease (arthritis, tendonitis), but after resting for a couple of months (no-one to climb with during summer) it seems to have calmed down, so l am now ready to begin again.
Hence, my advice is, start again on the low grades. l know how tempting it will be to push yourself & go back to ur old grade but don't! It will seem like a waste of time, but do a few sessions of a grade below to build ur strength back up & get ur fingers used to it again. Do leads that take a lot of hanging around but with bigger holds & build up ur body tension & stuff again, instead of trying to do really crimpy climbs really fast.
l used to be pushing for 6b+ but I've just done a few sessions doing 5+ & 6a to try & gently reintroduce my fingers to climbing.

You could also try doing a fair bit of traversing (your climbing centre has a traverse wall right?).

NB this is all advice as applied to indoor climbing. l find this is much more intense than outdoor climbing as once you've finished a route u go immediately onto another, so u do not get much of a chance to rest. if you'd prefer to do it outside it'd be a much more gentle re-introduction anyway, given the time it takes to set-up, etc!

hope this helps.

Warm up a LOT on lower grade climbs than you used to be able to climb and you will recover your strength without hurting yourself. Do not try to climb your previously hardest grade until you know that your body is ready for it.

l have done this - warm up until you almost exhausted and you can not climb any more, rest for a while, then climb again. You wo not have enough strength left to hurt yourself.

Good Luck