This is probably the biggest issue of all because taking unders in any form of punting is a fast track to the poor house. Look at the pre-post markets and they are normally framed to around 175%, yet on the day of the race you'll be able to shop around and bet into a market as close to 100% as you'll ever find.
To give you an idea of how heavily a 175% market is weighted in the bookies favour, it's the equivalent of taking $1.14 each of two in a two-horse race. Or $1.70 for each runner in a three-horse race.
So even if your early pick makes it to the first Tuesday in November it may actually start at longer odds on the day and you would have been better off waiting. All that risk for a negative return!
By all means have a couple of dollars if you already have a real fancy in the Cup, but save your real betting for raceday.