Some good advice all-round from people here. Some 'moons' ago when I was daft enough to compete in Triathlons I used to practice the 'transition' from cycle to run (allways a drama with co-ordination after long bike rides) by riding fairly hard for 10-15 minutes then jumping off and running for as long as I felt comfortable with (pushing the bike along by the saddle). This routine has now become a favourite work-out, you run till you've had enough then take a 'breather' on the bike. How much effort you apply is up to you.
Stay safe,
A. Berk
