Futsal is a very "quick" sport. Usually players has to run a lot (like in basketball). That's why this kind of sports has a big rotation of players on the field.
If you are playing friendly, the last defender could be a nice position, only covering your own half and helping attacking from the center of the field.
If you need a rest, being a goalkeeper is the easiest position to rest, since you barely leave the penalty area.
That's always depends on the type of game you are playing. If you are playing in a league, you will have to make sure your teammates doesn't fill all the positions you can handle. Or make sure that the team has replacement players. If you have a good goalkeeper (this position needs a lot of reflex, the ball in this game goes very fast), then stick to be the last defender. And start training to regain your fitness.