Training & Nutrition - Either I'm getting old or I just hit a limit...

18 days solid on the bike for a total of 700km
For those 18 days I've cut back on my food intake by half.
I get up at 5am to be able to get the time to go so far.

Yesterday I pushed for the fastest 50km I've done in a while. Most of the way was at 27 km/h which is fast for me on a mtb. Felt great all day after that. Did my 4 hours of song and dance routines at Kindergartens - still felt great.

Today my legs were complaining about the fast 50 so I took it really slow and did an easy 25 in the morning. Did another 4 hours of song and dance routines to edutain the kids and ate enough lunch to allow to function, but stay hungry.

But, I think I've hit some kind of wall because I barely had the strength to ride the motorcycle home. I got away with this routine for 4 months solid 4 years ago, but it seems that age is slowly catching up... :(

You may be getting old, but that probably has nothing to do with it. You're tired. Perfectly normal. And while your distances clearly aren't excessive, 18consecutive days training probably isn't a great idea. To get fit you have to give your muscles time to recover and adapt. And lots of that happens while you're asleep; if you're getting up every day at five I don't imagine you're getting 7-8 hours sleep per night, which you need when you're training.

Plus, controlling your diet is essential but halving your intake? Unless you were pigging out every day, that is extreme. In fact, extreme is the word that occurs to me from the tone of your whole post. If you want to improve your fitness and reduce your weight in a way that you will be able to sustain indefinitely, adopting a rather more gradual approach might be a good idea.

It might indeed. But I haven't noticed any flagging of energy until today. I make sure to sleep 7 hours a night by getting to bed earlier. I try and limit the distance in the morning to less than 40km to allow me to have enough energy for the rest of day. I've found doing that I feel I have more energy rather than less and I feel a lot better. Perhaps I was pushing too hard yesterday. I think a rest day tomorrow followed by however far I can go in 4 hours on Sunday.

As for food, wise? no. Extreme? I'm not trying to win any races, just get rid of the weight that has accumulated over the 2 years I got distracted from the bike. 2 years ago I did something very similar and dropped 11kg in 4 months. It was tiring, it hurt, but it worked. Sadly I injured myself in a unrelated airsoft accident that put my calf muscle out of useful action for 3 months. Then it turned winter... and thus 2 years passed.

At any rate I'm determined to do whatever it takes to get back to 85kg as quickly as possible. The fact that I have a new bike that is so easy to ride its silly makes it even more tempting to ride. :)

11kg in four months sounds perfectly reasonable. But it doesn't sound like someone who halves their food intake. If you want to avoid yo-yoing with your weight, I'd strongly recommend that your goal should not be to get back to 85kg as quickly as possible, but to stay at 85kg when you get there. You are more likely to do that if you make smaller changes that you will be able to make permanent.

Well that easy day made a huge difference. Despite feeling bad yesterday, today was the fastest and easiest I've ever done. Go figure. Feels like someone pushed the "level up" button. Cruising along at 29km/h felt like moderate exertion at most and set a new speed record (for me) of 45.5km/h. Guess the extra stress from pushing a bit beyond the usual helped.