most important thing for this length of race is a decent warm up
i wouldn't be put off longer races, just run them with a realistic expectation.
the problem with races is that they can have a negative effect if you finish further down the pack than you'd like - but you could be against runners for whom this is the biggest race of the season/have time to train/are naturally suited to fell running. Couple this with your own personal factors (fatigue, a recent long run still in the legs) and it's easy to get disheartened.
just enjoy your time on the hill, if you want to make a target for yourself then aim to be 1 minute quicker next year!