I think it's fine to use the tracker to find out how much walking you're actually doing, and to keep track of your total over time. But like you say, it's probably not a good idea to add all those calories into your recommended calorie range. Both the Spark tracker and any other online exercise tracker will probably overestimate the number of additional calories you actually burn with the walking, partly because the trackers assume this is "exercise," while for you it's something you do all the time, which means your body has adjusted to it in a way that will reduce the calorie expenditure by quite a bit. Our bodies do that with anything that we do very frequently, getting very efficient at doing it will less energy expenditure--that's why it's important to make sure that with your formal exercise, you're always challenging yourself to work a little harder than you have before.
So, when it comes to eating, I think the best bet would be to figure you'll need to eat a little more on those long workdays, but use your appetite and energy levels as a guide for how much, rather than trying to use the calorie burn numbers from the tracker.
Hope this helps.
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