I would also ask what you have been monitoring your heart rate with. If it's the heart rate monitor on your equipment they are often inaccurate.
I've even had issues with 'gym quality' HRMs on equipment at the gym.
I usualy prefer percieved rate of exertion. Go hard enough to breath sort of hard and sweat some, but don't push yourself to puffing, gasping exhuastion. You should be able to hold a conversation. And I don't mean " yeah...I've been...trying anew...eating/ exercise....protocall"
Conversation, not talking between gasps.
Please find out what your numbness is, in case it's dangerous.
I'm not old, I'm Vintage!
I would agree with the others about getting the numbness checked out, just to rule out a physiological problem other than the particular exercises that you are choosing to do being the cause of it. That being said, the fact that the numbness occurs when you bike or walk sounds like a circulation problem that may well be connected to musculature. When you're sitting on a bike, especially if you haven't for a while and are heavy, it's a common problem. I weigh about 200 and have had the same exact problem when biking. A solution for that might be going to a bike store and purchasing a different type of seat, or a seat cover with gel cushions in it to install on your bike.
As for the walking, again...having medically ruled out any significant physiological cause, it could be a matter of tight glute muscles. There are several stretches that you could do prior to and then after walking that may benefit you. Google glute stretches.