I'm not sure on the details of a Trek 520 but I raced an old touring bike for years with a single 42 ring and a single 8-speed barcon. In fact I made it up to a cat 2 on that bike. Now I have 10 speed STI and quite frankly I don't like it better then the single barcon in friction mode. With the barcon I could charge into a corner full speed and dump the whole cassette in one shot to be in my lowest gear for a slow haul. Ten speeds in cross is kind of silly really. Most of the cogs are just used to move the chain over to the 3 or 4 cogs you will use on an entire race.
Just my 2 cents. I think you'll be fine on an old touring bike.