What is this oddly notion you have that guinea-pigs exercise ?
My Lardy Sow will send you a bar of carbolic soap forthwith to wash your mouth out ! 
We had our GP6/GP7 in a huge C&C indoors with a hay area , tunnel, cardboard box . They pootled about happily going from bowl to hay to tunnel.
As long as they can mosey in and out of boxes and hunt down food , they will keep themselves busy .
It is lovely to see a piggie popcorn in the grass when they have their first grazing session
Now we have just GP7 , we've given her the plastic based cage (its 4'x2' ) and she has a tunnel and hay pile with a cover over half .
I'm planning on giving her a C&C run downstairs for day time (she's in the small bedroom at the moment , there's no door on it and DD is usually in her room) .
She's only been alone for a week so I don't want too much change for her just yet.
She will just sit still on hard floors too, they don't seem to like laminate or tile.
In the Summer , I'm going to put the rabbit hutch in the run (rabbit hutch has no legs ) so she can go out when we're home . I;m not putting her in the Pighouse on her own.
GP7 is the last of our pigs and I wouldn't want to risk a PigMate. She didn't like GP4/GP5 , used to chase GP8 (her cagemate) sometimes and only allowed GP6 to marry her when GP8 died (and I was dubious if she'd let him but they settled fine)