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It does help to do a HM as part of your marathon training. The timing will depend on your training schedule and your aims for the HM.
What training will you have to forego to take part in the HM race? If its one of the key long runs, then can you re-schedule your plan (e.g. swap two weeks around) so that you can still do it?
Are you planning to use the HM as a training run for the marathon, or do you want to race the HM with the aim of achieving a PB? If its the latter, this will have a detrimental effect on your training for at least a week after the HM event, so you'll need to take this into account.
With your training schedule, do you have a three-week taper? If you do, you may want to avoid this period. If it helps, the schedule I'm following (Pfitzinger & Douglas) suggests tune-up races at 4 and 6 weeks before the marathon. so I'd be happy to race on the 24th March but would be nervous of doing a HM any later than this (but thats me!).
Just back up the above. It depends on your self discipline. If you do the HM as a training run that also gives you a bit of "big day" experience... then fine. but make sure you don't get carried away and start racing like hell!
If you want to do a fairly decent pace at HM, then I think you need 6 weeks between HM and Full Marathon - or you'll exhaust too much of your resources on you HM, and not be able to train optimally and be ready for the Full.
thanks for the advice, i was going to use the half purely as part of my training for the Londdon marathon, get some feel for race day conditions etc, will avoid anything within 6 weeks, thanks again!
I ran VLM last year and my last half was around 15th March. I ran this at my marathon pace (well inevitably i got caught up so there were times I was running at 8:45 pace instead of 9:00 ooops) but i was fine on the big day. It also gave me a chance to have race experience of using my gels and gear (although I had been practising on long runs) and also helped me to work out my breakfast strategy.