How long before exercise causes weight loss?

You need to seriously address your impatience or it will most likely cause you to give up as it will undermine your confidence in the process.
Stop looking day to day and look ahead in terms of months and years.

For an extreme example I did a 9 hour cycle ride and ended up with a c. 1800 calorie deficit as just couldn't eat enough. Next day my weight was up a couple of pounds. Weight and weight fluctuations aren't just fat.

Exercise is for health and fitness benefits (and hopefully enjoyment!) - it's not "for nothing".

Exercise does not in itself cause weight loss. Eating fewer calories than you burn does. So you can exercise all you want to - if you eat too much during the day, you will not lose weight. On the other hand, you can lose weight without exercising at all. Also, patience young padawan - weight loss takes time.

Exercise in itself doesn't cause weight loss--eating less does. Watch your intake if you want results.

Exercise will not induce weight loss. Weight loss is about diet. To start seeing changes you need to eat at a calorie deficiet over a long period of time.

Nothing, if you out-eat the calories burned.

Exercise has been the most enjoyable part of my weight loss. Not for the scale, but for fitness, mobility, energy, flexibility, vitality, and strength.

If I’m not losing weight for those health improvements, what is it all for anyways?

You can't assume when you weigh yourself in the morning that whatever change you see was specifically caused by what you did yesterday. Your body is constantly storing fat, burning fat, repairing muscle, digesting food, releasing and attracting water, etc. Myriad things affect which of these things is happening and how fast it's happening. There is no way to connect one measurement on the scale to one cause. Weight loss/gain takes months and years, and weight management takes forever. You need to start taking the long view.

Also, exercise doesn't really "cause" weight loss. It can contribute to your calorie deficit, and that will over time cause weight loss, but only if your eating is also in line. :drinker:

Calories for weight.
Exercise for health.

Eating at a deficit is what causes weight loss. Exercise may help with remaining at a deficit, but in itself doesn't cause weight loss.

Wow. Ok, I am not saying this to be mean, but to honest and hopefully, helpful. If you are expecting to see a noticeable difference in your weight from one 10K walk, then you need to seriously re-evaluate your expectations and your plan here. Weight loss not only takes time and patience, but it also takes a bit of faith. Faith in knowing that if you stick with it, you will see the results you are expecting, it just isn't going to happen over night. We live in a world of instant gratification, which is why so many people fail at this. Weight loss isn't about instant results and there is no way to make it happen that way. You have to develop a plan of eating and exercise that you can sustain and actually enjoy because it is going to take time. If walking 10K feels daunting and leaves you sore, then try walking 5k instead. If eating 1200 calories per day leaves you hungry and grouchy, then eat 1500 instead. The point is, if what you are doing seems too hard, you aren't going to stick with it. Make it easier on yourself and be patient. It will happen, it just takes time, patience and persistence. And reasonable expectations. Wishing you the best of luck!