Heart Rate Query (possible overtraining?)

I started cycling last April, having not cycled for about 15 years - and even then it was only a paper round so probably doesn't count. A year on, I've done 7600 miles and I'm 3 stone lighter (11st 6lb) and now fairly regularly averaging 19mph+ for the commute to work (20 miles each way).
Until recently this 19mph+ effort would have given me an average HR of 170+ (max 192) ... but recently it's dropped down to 153-160bpm (even one of 148bpm recently), not that I'm complaining. I used to be relatively comfortable riding between 170 and 185 bpm, and could sustain this for over an hour at a time, but now when I hit those HR figures not only does it take alot more effort to reach, but my legs tire a lot more quickly than they used to and breathing is more laboured than I remember.
Could this be a sign that I'm overdoing things, and need to take a rest (a week?). I do take one-two rest/recovery days a week but perhaps this isn't enough - especially as I've upped the miles from 750-800 miles to 920 this month (Strava Classic Challenge).
Until recently this 19mph+ effort would have given me an average HR of 170+ (max 192) ... but recently it's dropped down to 153-160bpm (even one of 148bpm recently), not that I'm complaining. I used to be relatively comfortable riding between 170 and 185 bpm, and could sustain this for over an hour at a time, but now when I hit those HR figures not only does it take alot more effort to reach, but my legs tire a lot more quickly than they used to and breathing is more laboured than I remember.
Could this be a sign that I'm overdoing things, and need to take a rest (a week?). I do take one-two rest/recovery days a week but perhaps this isn't enough - especially as I've upped the miles from 750-800 miles to 920 this month (Strava Classic Challenge).
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you certainly do not need a week off. maybe a day or two. it gets harder and harder to raise your HR the more tired you become during a hard ride, or during a hard weeks riding.
http://app.strava.com/activities/51555293
Must have something to do with most of the 900+ miles this month being in the strong winds ... bonus.