Missed 45 minute run

Skip it, or if you have a scheduled day off soon, do it on that day since you had yesterday off.

Probably what I would do.

Edit:

Let me back up.  If it were me, I would do it, if it was not a high intensity workout.  But 45 minutes is not that long for what I'm training for.

However, with no idea of your weekly volume, or your intensity levels, or your fitness level, I would suggest skipping or doing it on your day off like I said above.

Like the other posters, its just hard to know when you don't know the whole training program or your level of training. 

When I was training for a marathon, If I missed a training day, then it was just gone.  I didn't try to fit it in on an other day because I felt like that would mess up the future training schedule.  For example, I had Friday as an off day.  I wasn't going to fit in a missed run on Friday evening before I did my long run at 7am on Saturday morning.

I would say that unless you start missing a lot of training days, one missed one is no big deal.

Thanks for the replies.  Should have rememebered, "Don't fret the small stuff!"