Dude, Where's My Pace?

Went out for a run today, my first since my disastrous 1/2 marathon.

Today (8km):
Pace: 4:57/km, Avg HR: 162bpm

That's a bit higher than I would normally see at that pace, but consider...

1/2 marathon:
Pace: 5:12/km, Avg HR: 177bpm!

I still haven't been able to pin-point what went wrong... maybe it was just a combination of things.  I haven't felt 100% the last few days, a bit lethargic, so it could just be I'm fighting a bug of some sort.

Don't tell anyone, but I'm thinking of doing a 10 mile race this weekend.  It's local and I'm stupid.