It sounds like the issue are a combination of the target times I've downloaded from this website plus that I push myself on the runs I feel comfortable with, leaving me with little in the tank for the faster more intensive ones. I'll check the data I've used to generate the training plan times.
I'm also running 6 days a week which my body is still probably adjusting to.
Thanks very much for taking the time to read/reply.
I'm training for the VLM and using the Garmin sub-3.00 program. I first ran the VLM in 2010 using the sub-3.30 program and acheived 3:16:45. I don't recall struggling with the shorter distance, fast-paced runs back then. I between then and now, 90% of my running would have been brisk but comfortable 5M runs at c 7.15 minute miles.
I feel fit and healthy and I am hitting the top pace threshold for all my distances (very comfortably on the LSRs) but on the fast runs (eg. 8 x 800m at 6 min mile or 6M at 6.25 min miles) I just can't keep up the pace.
The wierd thing is that my HR doesn't get above 160bpm (I'm just about to hit the big 4-0 ) even though I feel completely out of steam and just can't push any faster. As a comparison, last Saturday I ran 16M at 7.30 minute miles and my HR was a steady 150bpm. I feel so motivated before the session and yet shortly after hitting the target pace I just know that I won't be able to keep it up and I gradually get slower and slower.
Could it be a fueling problem or a core fitness issue? I track all my training on Strava and always analyse my training runs afterwards and it just seems like I can't sustain anything faster than 6.45 minute miles for much longer than a few miles.
Either way I am going to keep with this training plan as all the other training runs I am bang on target. I've got a 10k to acheive this weekend at sub-6.30 min miles so I'd love to get that in the bag not least so that mentally I can get a bit of confidence back.