Anyone know a good cycling / walking programme?

[whatanothernamechange]

I have recently started cycling a lot. The cycling is fine and I recently did 16 miles in 1 1/2 hours, but today i walked 5.5 miles in the same time and my knees really seized up. I have noticed this recently when I do any walking that is more than normal, ie 3+ miles. They don't hurt, just get and feel stiff. any tips / advice?