If walking is your capacity it is a start. If someones capacity is pretty low Turkish getup with no weight would be a great strength mechanism. And may just save a life.
When a human loses the capacity to get itself off the floor the wheels come off as humans and longevity.
Personally look at the mechanism of how extended steady state cardio works.
http://fitness.mercola.com/sites/fit...t-dangers.aspx
walking lacks intensity so it is the duration is biggest measure. When adaptation to walking happens I wouldn't move walking farther or to jogging/ running. If you can walk a decent pace for 20 minutes I would move to a rower or airdyne (not a regular bike). Rower and airdyne have a strength component and are upper and lower body. The airdyne has the ability for constant push pull. The rower has a deload phase as you bring yourself in. I always did about 10 mins of low intensity cardio (walk, row easy, or light bike) to get the joints lubed before I started the circuits. It just kept it brief no longer than 15 minutes. Then I would do strength work or burst cycles like HIIT. And honestly it has to suck for while on the rower and airdyne or it just steady state. But it is over in 20 mins +/-. I believe I started with 30 seconds of work and 60 sec active rest. i didn't stop rowing or pedaling I just eased up to warmup intensity. I would do that for a specific number of sets then have a cooldown period 5 minutes. As my engine improved I increased work phase and reduced rest phase. 25 - 30 minutes done.
Crossfit is good also in my opinion I had to scale. Gravity sucks for big people. I could deadlift 400 lbs but couldn't do a pullup without a big rubber band to help. I did CF after moving to a new state and not having a outlet that matched my previous volumes. I was in transition of IP to paleo, I didn't want to have to start over with a new IP coach (mistake I was only phase 1). I did fine and felt great in CF. I was new to CF so a lot of it was scaled for me. But I had the previous engine to work with also.
I think a great bang for the buck even for those very untrained is push/ pull sled. Push pull sled helps all three measures below and doesn't tax the body like other forms of exercise. I seen people push empty sleds as a start so it is very scalable.
http://www.fringesport.com/collectio...n-prowler-sled
Cardio burns fat at that moment. When you wipe the sweat off the fat burning stops. Moving to a more muscle building component, is muscle building and fat burning. studies show a fat burning effect upto 24 hours after weight training through what is known as EPOC (excess post-exercise oxygen consumption). I know my choice.
A infamous quote from a training coach I like. 'Strong people are harder to kill, and more useful in overall.'
http://daily.barbellshrugged.com/3-t...-about-muscle/
3 measures of health in older ages who had great longevity
#1 leg strength
#2 good VO2 Max
#3 good healthy amount of lean body mass (muscle)