Hi Kerensa
I had the same problem with exactly that session - I'm a week ahead of you as I'm aiming for New Forest. The other sessions I've found fine, including this weeks, but that one I just had real problems with. As I'm running different paces (aiming for 1:45, so my 5K pace according to the table is 14.2km/hr (can't remember what it is in minutes)), I htink maybe it's just that that session is really tough.
I'd agree with Dustin, I feel that even if I didn't manage that session, just doing the speed session each week, has helped my running and I think will benefit me more than if I'd been doing another medium length run. But I'll find out on the 22nd!