The equipment you're using takes a lot of the shock out of your run, so "two-a-days" shouldn't prove to be much of a problem for ya. Especially on the elliptical. I don't know what level you started from or are aiming for but a damn good pair of shoes is more than essential; Sorbothane insoles (hell they may have been superceded by an improvement by now, for all I know) are fantastic for your joints from toe to spine. I advocate getting some and replacing the insoles that your shoes came with- take those and use 'em for your work shoes.
I did this when I was Active; about 5 miles a day, at about 265, and steel toed boots all the rest of the time and my feet never bothered me. Another thing most folks aren't aware of is that about 90% of your running shoes shock absorption properties is lost in about 150-250 miles, depending on your
weight, running habits, etc.
Just listen to yer body, knees especially and do what it tells you. Epsom salts is amazing for soreness too.
You joinin' the SEALS or sumthin?
Good luck!
EDIT: I almost forgot; running in water, if you've got the opportunity, is just so damn good for you it's almost beyond comparison, if [words=http://fleshlight.sjv.io/c/348327/302851/4702]stamina[/words] and MAX O2 is what yer aiming for.