I'd say go with how you feel and what you think you can keep up- theres no point setting yourself goals you will find it too much to keep up with a few months down the line. Make realistic goals.
Maybe work on your endurance- this can be done, especially if you are just starting to change and have just stopped smoking, things can only get better, right?
Something I do to get myself doing something which otherwise is too difficult for me to do are to un/jump/whatever it is for 1 min, then slow down for 1 min. Repeat till your satisfied with what you have done. When you get better increase your time/reps, when you run out of time, increase the work time and decrease the rest time. Eventually set out to run/workout without stopping.
Its important to remember your exercise regieme is only as good as your rest. When you workout you tear at muscle fibers, you wear out your joints, you tire yourself out. If you don't rest no only are you not as able to workout as hard when you come back to what you do, but you also can't repair your muscles or joints or anywhere else you worked out. Its the repair part which makes your body stronger and better for next time. Unles you rest, you wont be able to repair.