I would never had suggested such a trip for a first outing in the first place. You certainly weren't prepared for it. When you're already overweight and out of shape, taking heavier winter gear will only make the experience worse.

Since you say the steps put pressure on your back more than the legs (assuming uphill), I'm wondering if the pack is not a good fit for you or was not packed well so that may be something to look at, too.

But, yes, getting in shape is the first step. Going in nice weather so you can carry less would be my next recommendation for someone just starting out.