This is something that is quite frequently overlooked in the training department.  For cardio, I highly highly recommend buying a good pair of running shoes.  There are several reasons.

1) They are comfortable.  If you are going to be spending 45 minutes running on a treadmill, your feet should be comfortable.

2) They prevent injury.  This runs the gamut of several different things.  The constant hammering of running is hard on arches, ankles and knees.  Ive had plantar fascitis (inflammation of the tendon in the arch of the foot) and I will tell you this is something you REALLY don’t want, its miserable and has a tendency to linger.  I remember being about 15 years old and playing in the soccer state cup.  Do I remember the games where we actually upset a couple top ranked teams?…Not really.  Infact, the only thing I remember is how bad my foot hurt!

3) They are designed for cardio.  Basketball/tennis shoes run the gamut from great to terrible.  For lifting it doesn’t make much of a difference (though you could debate that on leg day) but for running why not give yourself the best chance?

My personal preference is Saucony or Nike.  I have owned just about every type of running shoe you can think of, but ever since I started wearing sauconys I haven’t tried another kind of shoe.  They aren’t the most popular or the most stylish, but they are reasonably priced and are extremely comfortable and seem to not need a “wear in” period at all.