At the end of last year, I (foolishly) entered the Luton Marathon and the Bedford Half (2 weeks apart) and then realised that there was a club cross-country on the Sunday in between. I wouldn't normally have run all 3 in such close succession, but didn't want to let the club down for XC - and really wanted to do Bedford. So I went out in the marathon with the mentality that I would 'enjoy' the run (or as I normally put it, "complete not compete") - but ended up running a PB. I then did some very easy runs throughout the next couple of weeks (apart from the XC), before doing the half and getting within 10 seconds of my half-PB (sub 1:23). I then took 3 whole weeks off - not from injury, but because I wanted to.
So I think it's possible to run a good half time after a marathon, but remember to be very good to yourself afterwards - no matter what the outcome is! I also wouldn't recommend doing it too often.
Obviously, this is just my experience - and it would depend on how injury prone you usually are (touch wood, I haven't had anything major for the last 5 years).