Walking is a good way to drop blood sugars as it is usually longish duration and using big muscles so your 2 options are:
1) eat an extra slice of bread or a biscuit before you go = if you're needing more than 20g that suggests you are going for a very long or hilly walk or perhaps that you might reduce your basal.
2) reduce your short acting insulin the meal before you go out. For example if you know that 1 unit tends to drop you by 3 mmol (average). Test when you start the walk and then again when you are back to see what has happened.
Whether you ate the extra carbs or dropped some insulin you will start out with a slightly higher bg with the aim of getting it back down to normal by the time you are back e.g. if I eat an extra bit of toast may get to 12 but after an hour's brisk walk I might be down at 7 or so. Ditto for the dose reduction. It isn't an exact science but if you stick to the same kind of walk each time you will get good at this.
The amount of carbs you need or dosage to drop will be individual to you so some experimentation may be needed.
Don't over treat it if you do go hypo or you will end up on a rollercoaster.
I love walking my dog but he's rubbish at fetching me a jelly baby when I need it !
Adam Brown has lots on this in his free diabetes book ' Bright Spots and Landmines' : He is a type 1 with a dog and cycles a lot.
https://brightspotsandlandmines.org/ (also on Audible and Amazon)