Thanks for the enthusiastic replies. Much appreciated.
Unfortunately I seem to be fighting a cold this week. I've rested a couple of days, but tonight's 4.3k felt tough.
I'll see how it goes, but at the moment I'm thinking the slightly more conservative approach might be the best one. Maybe I'll start at 7:40ish pace, and if that feels good at half way I will up it to 7:30, then as fast as possible in the home straight. I'd like to get 75 mins, but would be delighted with anything under 77.
I've recently been to a podiatrist about a toe problem, and got orthotic insoles. He said one leg was about 1cm shorter than the other, but it might be functional or structural. I haven't been back yet to try and find out which.
I'm guessing functional discrepancies can be treated with exercise but structural ones will need more.