Ditto to all Ten's points above! What have I told you about stealing my thoughts? 
90 minutes ticked off for me today with 3 (last week was 2) miles at MP - again felt fine and my HR at the end of the MP effort was only a beat higher than last week so happy happy!
My polar web analysis makes interesting reading - you get a graph system where:
green =You are recovered from previous training sessions and ready to train more. If you're continuously in green you can increase your cumulative training load by adding more training sessions to your weekly plan or making the training sessions more intensive. Intensive training sessions and races should preferably be done when you are "in green".
yellow = Cumulative training load is on a high level. You can still train but should avoid high intensity training and/or races.
And red =Cumulative training load is on a very high level. If you continue training when "in red", it may lead to a state of overreaching.
I have ascertained that you can swing from red to green within 2 days as long as you do an easy day after what it considers a hard day (either over 2 hours or involves efforts seems to do it). Luckily I haven't crossed the line marked 'training NOT recommended' halfway up the red zone yet...