Xtex,
If you are starting out from not too many miles a day - start out SLOW and easy and only increase distance GRADUALLy. (Max of 10% per week)
I was / am genetically lucky and have very good inherent aerobic capacity / adaptability - but that meant I could easily (in aerobic terms) increase mileage WAY more quickly my body (joints / tendons etc) could adapt to.
Over the years I have spent a LOT of time injured because I didn't increase my mileage SLOWLY enough for my joints / connective tissue to adapt to after a layoff.
As we age this becomes even MORE important - as our old bods don't adapt so quickly.
Another lesson I learned is that if you want to run big mileages without back problems get your abs / core REAL strong.
I found that running for long periods of time - my abs would tire, and this makes you run with your butt sticking out more (relatively) - this puts more strain on your lower back.
Exercises like the plank. and its variants are good for ab / core strength / endurance - I currently am holding this basic one shown below for 4 minutes - some of my less fit friends cant do this for more than 20 - 30 seconds!
plank1.jpg
(I'm a climber as well - we need good core strength - so I work on it!)
Also do as many of your miles on softer surfaces as you can - ie NOT on tarmac / sidewalks. (sounds like you are already doing that). Lessens the pounding on your joints.
You might also want to look at this too - it DOES make a difference - but it takes REAL patience to do it right!
http://www.markallenonline.com/Base.asp
Lastly and most importantly - ENJOY IT!
Good luck
Satori
xxx