You are in the right place.....

the idea of finding a vintage mountain bike is solid.

also start slow...so you only go x minutes or x distance the first day....just do it and before you know the minutes and distances will increase.

another thing I think is key is that you should select lower (easier) gears so that you pedal faster, not harder. better for the knees


And when you do go out, you should be proud. you are taking charge and changing......