newbie rider needs help

Quoted message said:

Hello fellow bikers!

I'm new to mountain biking and I need some help. I play
racquetball 3-5 days a week and purchased a bike at my
local bike shop to help the stamina in my legs. I bought a
Raleigh M20, just a basic aluminum frame to get me
started. I was riding a mountain bike trail Sunday and
after about 3 miles my lower back started hurting. Could
this be from my riding position?

Yes.

Either go to a bike shop for some fitting assistance, or
read up on it (there are tons of articles on fit/position).

Also, the body takes a while to adapt to any new activity;
and learning better technique (like absorbing shock with the
arms and legs) will make a big difference.

First thing I'd check is saddle height, and many newbies
seem to set it way too low.

Bill "my back aches, too, but I'm old" S.

A big yes sir.

I MTB 2004