Ah, yes, the great marmite shortage even made the news here

I visited with semi-distant British relatives a couple years ago and they had some, so I tried it. I actually liked it. But I like salty things in general.
Regarding going out, one thing I did was take a pillow with me to go to restaurants and such. It made the seats softer, and raised me up a bit so it was easier to get up and down. Also, at three weeks, I found an hour of sitting was the most I could do, although getting up to walk around periodically helps. Good for you for getting out a little bit! I think at three weeks I was still just doing some walking in my neighborhood. That was plenty!