This is precisely why I am trying this approach. I have been training pretty hard for about 2 years now, and have seen some pretty good gains in my fitness.
In my last 5k race my average HR was 191 which I reckon must be nearing 90%, and I can do a 10k at 185bpm.
Unfortunately this doesn't make me particularly fit (5k pb is 25mins), it just means that I have a high LT.
So I am happy to spend a few months trying this 'base training' lark. If all goes well I will be able to get my LT back up pretty quickly, and get some good improvements in speed.
The worst case scenario (as far as I can see) is that my fitness will stay at the same level, but I get 3 months of easy training :-)