Exercising for 45 minutes three times a week - enough to raise your heart rate - is the standard quantity to stay in reasonable general cardiovascular health - whatever your weight or fat level, whatever your walking speed (to some extent). If just walking exhausts you, have you sought medical advice?

Try 45 minutes of walking instead of two hours.

DDR can be done with glasses on, if they won't fly across the room ;-) I'm afraid "I can't do DDR without Lasik first" comes across as a mere excuse.

Yeah, that kind of advice is not going to fill any procedural knowledge gaps, sorry.

It is possible you may be generalising from one example here. nazgulnarsil's post struck me as actually a pretty good start: it doesn't matter tremendously much what you do as long as you do stuff.