Help! Leg cramps and stamina?

I have been doing the treadmill and stationary bike now for about 2 months about 3 days a week and then 2 days a week of weight training with a personal trainer.  My trainer, my Endocrinologist and my Dietitian all want me to up the intensity and length of my cardio. . .okay fine, no problem. . .except, if I go faster than 3.5-3.8 mph on the treadmill, I start getting muscle spasms and numbness and tingling in my toes and up the sides of my feet and calves.

The elliptical absolutely kills me, I apparently have NO stamina for that, which kind of surprises me. . .my thighs burn and my heart rate skyrockets . . .the burning thighs I can push through on the bike with no problem but the elliptical kicks my butt every time and I can't manage to do more than 2 minutes, which is really discouraging.  Especially when I look around and there are people literally twice my size going to town on these machines like it is nothing, and I am standing there huffing and puffing like a freight train, trying to figure out what the heck is wrong with me that I can't do it without feeling like I'm gonna kick the bucket!  

I just saw my cardiologist, and other than having a heart murmur and mitral regurgitation, which is not new, there is nothing wrong with my heart and I am cleared for exercise.  I am having an echocardiogram in about two weeks just because this is a new cardiologist and I haven't had one done in about 2 years and he just wants to take a look at my valves for himself.  He knows and has worked with my previous cardiologist and I was only one a once a year follow up schedule just to check in and make sure there aren't any changes and he said that with my blood work, other than my iron being low came back fine. 

I have neuropathy from diabetes and arthritis and I am on meds for both, but I am wondering if its normal to feel like that when you're starting off on the elliptical or if I'm just still really THAT out of shape and low on endurance and/or stamina!!! I'm actually kind of embarrassed to ask my trainer so I figured I would ask you guys.  I want to start the Couch to 5K but if I can't go over 3.5 mph without the muscle spasms, I don't know how I'm going to do that?!?!? I see my Endocrinologist tomorrow so I am going to talk to her about that in case it is diabetes-related and see if she has any ideas.

Anyone have any suggestions or similar experiences?  Thanks in advance!

I had similar cramping, and they found potassium levels were low. So with the combination of increased potassium intake and compression sleeves I all but eliminated this. 

Thank you!  I discussed this with my Endocrinologist yesterday and she recommended compression stockings and also that I may be slightly dehydrated so she wants me to start drinking extra water before I work out and see if that helps.  :)

Many times, if it is a true muscle cramp, it is due to low electrolytes, namely sodium. If I ever cramp (and I always try to keep up on my electrolytes during multi hour / multi day races) I drink pickle juice or take a packet of salt down and bingo... Problem solved. Of course, electrolytes are made up of more than sodium, so, as stated above, potassium, sodium, magnesium, etc are all important. Best of luck with everything...