When I've been out on the bike at the weekends recently I've been finding I can't push much beyond 3-4 hours. I feel fine up to this point climbing/descending etc, but after three hours I can't climb anything without getting massive lactic acid burn in my legs. I can keep pedalling on the flat and downhill is no problem (standing and handling on technical descents is fine). Same story when out on my road bike, hit three hours and my legs start to go. I also start to get cold about the same time, but I'm not sure if this is a cause or effect - get cold, blood doesn't go to muscles so much, get pain or get pain, can't work muscles as hard, get cold....
I want to get this sorted as I'm planning on doing the Tour de Ben in September and know I will die about half way round unless I fix this! My question is if there's anything I can do nutrition/warm up wise to delay the onset of this, or is it just a case of MTFU and exercise more/longer periods? I haven't really been getting the chance to do much during the week recently but will make the effort to get out in the evenings if that's what's required.
I've tried googling but just get info about post activity muscle soreness, but I don't get any of that either after my ride or the next day. Prior to this most of my riding has been no more than 2.5hours round cannock or the local bridleways.
Apologies for a slightly rambling post, I suspect the answer will be MTFU!

