Uphill struggle

I have recently started swimming, in an effort to become fitter and stronger. And with that in mind I have been walking too, but I have found my energy levels very depleted after walking uphill, and I had lots of the classic GERD symptoms popping up immediately afterwards, which I did NOT experience with swimming...this was unpleasant, I felt like an old person walking uphill, is this normal?.... I think I will just have to stick to swimming in future, or maybe I just aren't very fit at the moment and it will take a while to get my strength back..I just wondered what other peoples experiences are with walking, I live at the top of a mountain, so the air is different too!

Thanks a lot!..I think I have discovered what the problem was!

I was walking about an hour and a half after my breakfast! and I have realized it doesn't really work for me, yesterday I did not eat anything for breakfast and went out for the same walk and I had no GERD problems! I just had my breakfast when i got back home, also I live on the top of a mountain and I am walking uphill, so the air is thinner, plus you are correct, I have avoided exercise for a while, so I think a combination of all of the above is why I felt so bad...I feel really relieved actually!