Piman Like CW said, my hamstring, thigh and calf niggles all seem to originate from nerve problems. If in doubt, leave the fast stuff out. (Gobi once advised me on this, but in capitals and more industrial terms.) I think you can work on the pace later and over a shorter period if necessay (whereas endurance needs more consisteny miles.)
HR I don't think there's a problem with racing a half three weeks out, as long as you're not carrying a niggle. Could be a big confidence boost and if it goes well or you pick up a minor niggle, you have three weeks to recover. Sometimes we have to burn a couple of matches, even if we have a toaster at risk.
Lev_ Your build up sounds pretty good to me. It's rare not to have set backs, and you're already four weeks back and into good mileage with four quality weeks to come. (Hope I've got that right.) It may be that I'm seeing a glass half full, whereas we tend to see our own training in a more pessimistic light. I'd be a bit worried that by aiming to put the icing on the cake with an extra three weeks you may go past a peak and miss the opportuniy of the extra push London gives most of us. Anyway, as Padams said, it's too soon to set firm targets.