Re: Love training, and I have a heavy build and joint issues.
Welcome and perservere with the training. I am 6ft 5 and of big build and have been training for 14 years and initially had various aches and pains. I would agree with your sensei just do as much as you are comfortable with and take it slow and easy. You are in it for the long term.
I would recommend hatha yoga which is gentle and will help increase your flexibility (it was of great help to me and I went for a year). If you do not have access to a good yoga centre just try gentle stretching every day. Resting your joints and muscles is equally important as it allows your body to adapt.