Actually, Marc Singer is still in pretty good shape, IMHO. Indeed, I think most of the main cast still look pretty fit. Singer's only in his 50's, and I think the female cast members (Badler, Grant) are in their 40's. Not sure about Ironside, but he doesn't look much different. Just a bit greyer.
Oh, and I think you mean Richard Herd, not John Hurt I doubt we'll see the character of John back anyway, given his fate at the end of the Final Battle.