Can you drive? if not its not really a problem, trains run to Plymouth from Exeter all the time. Anyways best bet would be to go to plymouth on your mtb when theres a ride on, usually during the warmer months but they do crop up, such as the xmas eve ride. Then you'll get to try lots out... Everyone who rides plym is good enough lad, they'll have no probs with you trying their bike out for a bit.