Yes my event is in sept. I have changed the tyres... and that made a lot of diff...The Pendleton is a heavy(12kg) wide tyre hybrid.
Did you change tyres to narrower, smooth tread? The tyres are 26" diameter. These would be much faster https://www.bike24.com/p2197004.html if you haven't swapped tyres yet.
A road bike with drop bars would also increase speed by immediately making you more aero position, as opposed to sit up hybrid.
If I remember your event is in September? There is time to swap to another bike, but you need to make the decision soon so you can adapt to it. Cant you borrow/or buy with trial to see if you like a road bike
This bike https://www.rutlandcycling.com/bikes/road-bikes/specialized-dolce-2019-ladies-road-bike-grey_379972?currency=GBP&gclid=Cj0KCQjwocPnBRDFARIsAJJcf94GPPDYUCTkvKgehvJRo_xZg6Wdh_a2oA7NzjPczgoHZndtw10hTzoaAlc6EALw_wcB&gclsrc=aw.ds
Comes in frames sizes for 4'11" 5'1" They have 30 test ride policy(check details) Sold elsewhere.
If you can ride London to Brighton and didnt feel like death at the end there is nothing wrong with fitness/stamina.
Advice for speed training, ride hard for 1 minute, slow down to recover and repeat until you can do any more. You should be able to get upto 18-20mph for a minute. Its not about the actual speed just the pushing above your cruise limit. Use a fast-ish cadence 85-95rpm. this trains your legs muscles and nervous system to be fast reacting.
Yes london brighton was al right. Actually didnt feel any tiredness or cramps after that . But there was no time pressure ... i finished it in 5.43 hrs.
For l2p there is finishing times each day and hence i am worried.
Will try the cadence training that sounds good