Since his top speed varies so wildly from author to author and continuity to continuity, there's no way we're going to be able to pin down a "comfortable" run speed with 100% precision. It's hard to imagine why an author would bore a reader with his ho-hum speed. Usually, numbers are only going to be used when talking about a feat that's crazy fast, even by the Flash's standards.
All that being said, we should still be able to calculate something reasonable.
- Top Speed: 15.9 mph
- Jogging Speed: 8.3 mph (52.2% of top speed)
Based on calculated speed:
- Top Speed: 8,937,608,400,000,000,000,000 mph
- Jogging Speed: 4,665,431,584,800,000,000,000 mph (52.2% of top speed)
Based on written speed:
- Top Speed: 670,616,629 mph (speed of light)
- Jogging Speed: 350,061,880.338 mph (52.2% of top speed)
Your question has 2 parts:
what is the standard speed (broadly speaking) that the Flash uses on a day-to-day basis (or the range of that speed)
what speed does it become an effort or strain for him to maintain?
First we need to establish a baseline with a normal/average person. I've seen several exercise websites list a good treadmill jogging speed as between 5 and 6 miles per hour (such as this one and this one, for example). I tend to trust high-profile news sources more than I trust exercise recommendations from the internet so I found this article from The Telegraph that says:
The average man jogs at a speed of 8.3 mph
We'll go with that.
The same article gives us a sprinting speed of 15.9 mph.
So, we can use these numbers to say that an average male has a standard, maintainable speed of 52.2% of their top speed.
To extend this calculation for The Flash, we need to know his top speed. The fastest speed I've been able to find is from this image:
We have a discrepancy here because the author clearly notes the speed as just short of the speed of light but further inspection of the feat itself proves otherwise.
Based on the above calculation, The Flash can travel up to 2,482,669,000,000,000,000 miles per second. Converted to miles per hour, that's 8,937,608,400,000,000,000,000 mph. That's unimaginably fast...
Given that top speed, we can estimate his comfortable jogging speed at 4,665,431,584,800,000,000,000 mph, or 6,956,927,972,032.14 times the speed of light.
If we allow the author a little breathing room and place The Flash's top speed at the speed of light (670,616,629 mph), that puts his comfortable jogging speed at a much more modest 350,061,880.338 mph or 456,244 times the speed of sound.