I'd try and work in something to challenge lateral stability more. If you can work in some exercise of the abductors/adductors, especially if you can do it on the stability ball then you'll help protect against kneecap tracking injuries and ITB issues.

Make use of the stability ball for core strength training as well, it's arguably more important than leg strength which is easier to train out on the fell. Core strength and stability is what keeps you going in the longer events, without back pain and tripping over your feet!

If you are going to do some free weight stuff for the upper body then I wouldn't bother with bicep curls; it's a fairly pointless exercise solely for gym posers. I'd do some rotator cuff exercises; shoulder abductions and internal/external rotations (if the gym has a 'cable jungle' type machine) and maybe some shoulder shrugs and high elbow rows knelt on a bench.

TBH (and I know this goes against the title of your thread!) I really wouldn't waste much of your gym time doing leg strength training; better to do it out on the fell. There's very little in a gym that replicates the muscular endurance workout provided by a 2-3 hour hill run (unless you're going to spend the whole session on a stepper!). I do a 2 hour gym session twice a week and the only leg strength exercise I do is for the glutes and for the abductor/adductors as I mentioned above; takes about 10-15 minutes. Most of the session is based on a stability ball or a wobble board and it's pretty tough.