If there's not too much wind it shouldn't be a problem. In fact we're considering adding longer TT's (probably a 40k) to the ones we're doing at our Speedway (only a 10 mile event now) so hopefully I'll get the chance to find out sometime this year. I can comfortably do 23 1/2 mins in a 10 mile TT on a flat track so getting in under 60 mins for 20 miles shouldn't be too bad. Finishing under 60 mins for a 40k (just 4 more miles) might be a different story though.