Very sensible to set a limit on your heart rate, I've been recommended by stroke consultants to keep mine down, but unfortunately to a level that would barely allow me to pedal.
From medical advice I've received, it seems that a very good, gradual, warm up is important, as it can reduce strain on the blood circulation and blood pressure. As I understand it, when you start cycling you make demands on the blood supply to deliver oxygen so the heart rate rises to provide more, but if this is done before you warm up and allow the blood vessels to dilate then forcing the extra blood down the not fully dilated vessels causes the blood pressure to rise sharply. Once you have warmed up properly, the blood pressure tends to fall back.
In April I managed to provoke a second cerebral bleed following a short flat out race with a pretty inadequate warm-up (less than 7 minutes) as I arrived only just in time. I very much regret not warming up properly. I can't be sure, but if I had warmed up for 20-30 minutes it might not have happened. I had previously done many 1 and 2 hour marathon climbs at high heart rates without any apparent effect, but they may have had a cumulative effect, I suppose.
Anyway, I just wanted to say take it easy, warm up as much as you can and stick to your guns on your HR limit. It's very hard not push flat out when in a close race, but coming second is better than damaging yourself and perhaps not being able to ride again (or worse).
I came last in the race in April anyway, so I couldn't even say it was worth it!
Geoff