Yes it's hard, but it's also great fun.
I'm an overweight, unfit 50 something and I did agility training and (eventually) competitions with William until he damaged his back 3 years ago. If Albert has a good sit stay that will really help you because once he's learned how to jump etc. you can leave him at the start of a course and walk to a point where you can call him over the jumps. Having a good sendaway helps a lot too. My plan was always to do as little running as possible and let William do it
Initially you won't really be doing much running about anyway as it will be all about teaching Albert how to use each piece of equipment correctly. If you enjoy it and keep going with it it's the best way (in my opinion) to build up a very strong bond with your dog. You have to be a partnership, with Albert reading your body language and you giving the correct signs and body language to send him the right way. When you both get it right it's magic
PS. William's back damage wasn't necessarily due to doing agility, so don't worry about that.
Hope you enjoy it