I'm 46.  I was not a runner in school but in college learned to run 3 miles for exercise purposes.  I ran off and on in my 20s and low 30s.  About 10 years ago I decided I needed to get in shape.  I got fitted with the right shoes and started running a bit but I also started swimming.  That built up my endurance and made running easier.  In the last 8 months I've started doing more strength training but I still try to run once a week or so and our current progression at the gym includes a fair amount of running, although it tends to be 200m to 400m interval runs.  34 is not to old to run but you need to get yourself checked out by a doctor if you are always hurting......I still race my 11 year old daughter.  She gets off the line quicker but I still beat her.  My 17 year old son who hates running more than I did at his age is deceptively fast (but still slow.....genetics).