lower back

Hello , Just wanted to share my story and get some advice/suggestions from you all ... I have a lower back(left) pain since a year and a little over a month got into water and its been awesome feeling since then .. if I skip swim then I feel miserable and the pain comes back. I started with gasping for breath at the end of 25 m pool and would be out of the pool by 150 m in the first week and then gradually it moved to 300 ( Wonder how some of you keep track of the laps , I started with Monday -breakfast done , lunch done , dinner done over for marking as 300 m done ) and these days i can go upto 750m with a three breaks. At the end of 700 I don't feel like stopping and want to keep going , but fear that I might be over doing and might burn out of this so I "treat" myself with another 50 m and get out of the pool and can feel the body buring hot on the skin( i swim in an indoor pool ) . I am 40 and all day i sit on a desk doing computer stuff for work .. and go swim after work .... I swim freestyle .. Any suggestions how to keep this going and not burn out and better on this .. I am learning on youtube the techniques to swim better and implement one at a time everyday .... I am in Prague, CZ. Thanks a tonn , Cheers Umesh