The Miami Marathon looks like a smart debut on paper: 6 ft of climb per mile, a big race-weekend field, and if you live in South Florida it's the hometown race. Every one of those things is real. Run the half. Don't make the full your first marathon.
Here's the math. A typical Miami race morning over the last decade is 68°F with a 62°F dew point. On the temperature-plus-dew-point scale our pace engine uses, that's 130 — a 2% pace penalty for a continuous effort. For a 4:30 first-timer, that's about 5 minutes handed to the weather before the gun goes off. And that's the average year, not the bad year.
A first marathon is already an exercise in managing the unknown: fueling, pacing, the wall. High dew point attacks exactly the thing a first-timer has the least of — margin. Sweat stops evaporating, heart rate drifts, and the pace that felt easy at mile 8 becomes a death march at mile 20. Experienced runners adjust on the fly. First-timers blow up.
Compare Houston: same pancake-flat profile, but a typical January race morning is 47°F — 86 on the same scale, a 0% penalty. The flatness you're choosing Miami for is available elsewhere without the heat tax.
If Miami is your city, use the weekend anyway: the half on the same course is a superb tune-up race, and 13.1 miles in that air is a manageable dose. Debut at 26.2 somewhere the weather is on your side.