At 2.5 miles, your ride IS the warmup, and what you may do before depends on the time you're willing to commit to the idea.
OTOH- one way to reduce strain as you ease into the ride, is to spin gears lower than you would otherwise for the first few minutes of your ride. I'm not talking about going crazy, just one or two gear steps lower. You'll have to experiment to find how much and how long, but I estimate that a few minutes should be enough, though I wouldn't be shocked to find out that you had to do it for 90% of the ride.
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