Long run

For the past few years, I have tried to do a long run (which I arbitrarily define as anything over 5 miles) each Saturday morning. Today I tried to bite off a big chunk, and go 10 miles according to the half-marathon training schedule for the race I want to do in late August, even though I've not done 10 miles since April 2009. Setting off at 7 AM today, the early part of the run went fine (though I found it mentally very challenging to think of that many miles yet ahead of me). But after the 5-mile point, I began suffering once again from the inexplicable but dramatic lean to the right that affected me two weeks ago also, and consequently decided to shut it down early (just shy of 6 miles) and bus home. Now I am perplexed about the race - not doing the 10 miles today is a serious, but not fatal, blow. However, I am undecided as to whether it would be wiser to just concede that at my age (54) and post-trauma condition, it is not realistic and thus I should not pursue the race (though I know that many older runners than myself compete, and, after all, severe though my accident was, it DID happen 18 years ago!); or whether I should still continue doggedly to pursue this goal.