“Walk” is the standard word for this action. If you’re not sure, use “walk”. “To pace” is much less common, and it has several specialized meanings: 1) to walk back and forth when you are nervous,bored, or trying to think of something “The writer paced in his room, trying to think of the next sentence in his book” 2)(with off) to measure a distance by walking carefully “We paced off the field, and decided it was about 130 meters long” There are a few other meanings, but they’re even rarer. In general,use “walk”.

To say 'I am walking' would mean that you are actively going somewhere, 'walking to the store' or 'walking to get exercise,' it's describing the activity of putting one foot in front of the other. 'Pacing' can indicate nervous activity, as in ' he is pacing while he waits for the phone call,' as in walking back and forth in a nervous manner. It can also mean to set and moderate speed, as in ' the lead runner set the pace of the race,' or 'I am eating too fast, I should pace myself,' which would indicate a desire to seta slower eating speed.

When a person is “pacing”, it means that they are nervously or anxiously walking back and forward. In normal circumstances, a person merely “walks”.

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