I rode a 70k recently. started to bonk at 3miles at got progressivly worse, deciding there is no way I could do the second lap (especially after being taken over by people in a lower category, children and fatties). I kept going, shit time and turns out my front brake was full on from the point i went over the cattle grid (at 3 miles) until I finished. I thought the hissy sound might have been a puncture but the tyres felt fine.
Think I'd better learn about bike maintenance before I embark on my next longish distance ride!