Hmm.
I would say walking, especially with load (backpack, bag) is harder than biking.
Why I don't like other opinions in here saying that biking > walking is because 2.5 miles is really short distance for a bike. I freakin run 5 miles for my cardio. When I bike, I bike for at least 15 miles.
Why walking is hard? Because every day, I walk 8 min to the bus stop, then a mile (15 min) from bus stop to college campus. With backpack. Then I walk back to bus (1 mile), and then 8 minutes to the door. Overall that counts to about 2.5 miles. While it is not particularly tiring, it is not a walk in the park.
So, OP, try walking. Biking 2.5 miles is a joke! Anybody in half decent aerobic shape would just finish warming up in 3 miles of bike.
OP, you can also bike to work and do additional cardio such as longer bike ride or running.
edit: plus, if you walk at my pace (1 mile = 15 min, pretty relaxed walk) walking will take you 38 min, while biking, even if you bike at slow 10mph, will take you only 15 minutes. So, when you walk, your cardio system will be more active for 3x longer (but less intensity than bike).
And I am skeptical about the whole "lasting effect on metabolism" thing.