Man I gotta agree with the being tired thing when you change up traiing systems. First I'll say I was never exceptional so please don't crack up over my times, but here goes.
I was a low 18 minute 5k guy my sophomore year in HS off of literally 15-20 miles a week. I then moved to a country hick town in another state, the cross coach there believed in bigger mileage. I thought he was out of his brain at the time. Anyways, we did anywhere from 6-10 a day in the summer. Nothing huge but a definite increase from anything I'd done prior. The weird thing was I was absolutely killing anyone else in practice on a daily basis. They were probably laughing at me in hindsight for running hard everyday like that. But given that, everyone expected big things from me once the season started. Hell I did too. Well, first meet I busted one of the worse times of my life. I could probably go out right now and run a faster 5k and I haven't ran in forever. 23+ minutes!!! I don't think I even felt so tried during a race either. Legs felt like wet noodles but I forced myself to finish.
It took me to one of the last races of that LOOONG season to hit a decent time by my standards. And another entire year in that same system to eventually run low 16's. I eventually got there but I think the increase was a real shock to my system. Sometimes it takes awhile I think to see results. Not sure if this applies to better runners but it definetely hurt a scrub like me.