Long excercise new on insulin

Sounds like you're taking your insulin in the morning, if you're going low at lunchtime.

Do you split the dose at all? The Humulin I peak activity is ~6h and it tails off significantly (https://www.medicines.org.uk/emc/product/8196/smpc#gref)

If you're already going low, and will be doing quite a lot more exercise than usual, you'll end up going lower if you leave the dose alone. You may be happy to combat this by eating more (for breakfast and morning coffee in your case, though likely more will be required throughout the day too.)

Even without the walking holiday I'd be tempted to split the existing dose in half (for example) to avoid the pre-lunch hypo, though do then do some monitoring overnight to prevent hypos then, which is perhaps why you've been started like this.

Exercise will increase insulin sensitivity, so if it were me, I'd think about reducing the total dose.

You may find that you don't go as high in the evening as you expect (from the food) as many people suffer from post-exercise low blood sugar (muscles and liver are replenishing their stores), though if you are used to the exercise you're doing this effect is less pronounced.