I do that. When spark guesses how many calories I would burn in the time I say I'll work out for, I never meet that many calories.
Stop Spark from guessing.
On the Fitness Goals screen you can include how many calories you actually burn per week. You can put in this week what you burned last week and adjust that as often as necessary, to be more accurate than Spark's guess based on your time.
Deb, in New Zealand