Hi Josephine,
I'm guessing from the picture that the Roadmaster wasn't an expensive bike to start with. If you're going to be replacing wheels, tyres, cassette, chain etc. you'll probably end up spending what the bike originally cost again.
If you're riding 60+ miles a week, have you thought about replacing the whole bike with a better one, you're doing enough miles to justify one.
You could also consider something more suitable for your rides, if your mainly on road, but don't want a full on racer with drop bars, consider something like a hybrid bike, this should be a lot less effort to ride on the road, but with suitable tyres will be happy on light trails and unmade tracks as well:
If you're looking for a good mixed terrain tyre, try Schwalbe Hurricanes:
http://www.schwalbetires.com/bike_tires/off-road_tires/hurricane, they're semi slick, so they roll well on tarmac, but the knobbly side tread makes them OK off-road provided it's not too wet or muddy.
Lastly, congratulations, dropping 65lbs in a year is good going. I'm on a bit of a weight loss regime myself, lost about 33lbs so far and reckon I need to lose another 30 to 40 to get down to my fighting weight.
Owen.