Over 30s hints please!

StewartMacLean110281

Hi guys, I'm 36, 6'2 and 13.5 stone. Been a regular 10k (pb 39:15) and half marathon (1:26) runner for about 8 years. Last year or so the knees have been not too happy with doing anything over 10 miles. Don't want to admit it's my age but probably is. Any hints to aliviate aches and pains??