I'm running in an old pair of Mizuno trail shoes (Wave Elixir 4 X10 if you need to know) from yonks ago (in my 2008 fitter days). I bought them from a professional store after some gait analysis. Problem is, they don't seem to be supporting my ankles/knees as much as before (or am I older???). According to Nike+ they've done about 500kms, a mixture of road and trails, but all road on C25K.
Would I expect more than that? Do I need new shoes?
Mine got knackered at about 700k ( Also via Nike +). I could tell as my shins started playing up again...worth getting proper gait analysis when you do.... 
I don't know about the distance but I would suggest that using trail shoes for road running will be disagreeable. If roads are unavoidable it may be time for some road shoes and you may find the trail shoes are good for a while yet on softer surfaces.
Most expertexperts suggest that running shoes should be replaced after about 500 miles.
Checkout the Runners Need website. They have a good page about shoes.
It could be as simple as the fact that you are using trail shoes on the road; they are not designed for road use and can play havoc with your legs.
Bite the bullet and get some road ones ~ you don't need this year's latest model, and you can save a fortune buying last years model.
Get them fitted by a decent running shop and you should be fine ~ discounting the fact that you are older!!
Good luck.
Running shoes should be replaced between 300 - 500 miles depending on how they have worn. I personally don't have experience of trail shoes but it is worth taking on board the other comments of getting some road shoes. My first pair of shoes were ordinary trainers (yikes!) and I was lucky to get quite a lot of mileage from them without injury but when I got my gait analysis done & my new beautiful running shoes - the comfort & support they provided made a real difference. Look at older models / past seasons as they will be cheaper.
It depends but certainly advice says shoes should be changed every 400 miles as a minimum and potentially up to evey 200 miles depending on the type of training you are doing.
Good running shoes?
My new pair of 'proper shoes'! Please help!
How necessary are running shoes?
How important are the running shoes ?
Running shoes are a rip-off
