Stiff legs

Afternoon.
The last 2 sportives that I've completed I've experienced a wierd stiff legged sensation that I'm struggling to understand the cause so I though I'd throw it open to you guys here. After taking the first few miles at a fairly steady pace to ensure I've properly warmed up, I increase the tempo and try to ride at just below threshold ( i.e the fastest pace before conversation becomes stacatto'd ) for the remainder. After about 35 miles, when i have previously exprienced no issues, my legs start to really feel stiff and achey and really hurt to try and put any effort through. I'm trying to ride at the same speed on the flats ( anywhere from about 23-30 mph ) and I'm struggling to sustain 18mph and my heart rate won't get above 65-70% max. It feels like I've pulled all the muscles in my legs, comparable when you wake up in the morning after a hard ride with no warmup or cool down. I ride to work every day on the same bike, around 15 miles in total. I try to make up for what I lack in endurance on my commute by doing a few intervals ( i.e Strava sprints .... ). It's only been the last 2 sportives that I suffered this and nothing else has changed.....
Anyone any ideas
Cheers
The last 2 sportives that I've completed I've experienced a wierd stiff legged sensation that I'm struggling to understand the cause so I though I'd throw it open to you guys here. After taking the first few miles at a fairly steady pace to ensure I've properly warmed up, I increase the tempo and try to ride at just below threshold ( i.e the fastest pace before conversation becomes stacatto'd ) for the remainder. After about 35 miles, when i have previously exprienced no issues, my legs start to really feel stiff and achey and really hurt to try and put any effort through. I'm trying to ride at the same speed on the flats ( anywhere from about 23-30 mph ) and I'm struggling to sustain 18mph and my heart rate won't get above 65-70% max. It feels like I've pulled all the muscles in my legs, comparable when you wake up in the morning after a hard ride with no warmup or cool down. I ride to work every day on the same bike, around 15 miles in total. I try to make up for what I lack in endurance on my commute by doing a few intervals ( i.e Strava sprints .... ). It's only been the last 2 sportives that I suffered this and nothing else has changed.....
Anyone any ideas
Cheers
FCN 5 - Fuji Roubaix 2
FCN 11 - Giant Trance X3
FCN 11 - Giant Trance X3
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it's a hard life if you don't weaken.
First I'd check under the seat to make sure the turbo button is still working.