It just depends on the luck of the draw. I've been there late November and even in December when the weather was fantastic...
and when it just sucked...
But yeah, in general it is likely to be on the cool side in a few weeks
There is a big difference between a 300 mile day and a 500 mile day if you are not used to doing the 500 mile days. If you are used to 300 mile days, your body knows this. Once you start getting much over 300 miles, your body will remind you of this. You will likely feel more sore and tired than normal. The chances for mistakes will go up considerably in that last 150 miles or so. I used to lead a lot of group rides and I could see this every single day of a weekend rally once we started getting over about 250 miles into a 300 mile route. People would start getting sloppy in the corners with their lines. They would be more likely to blow corners. As a result, I would always start backing down the pace after that first 250 miles. This is especially true if you are riding on roads that are twistier than what you are used to riding. They will demand more focus and concentration than you are used to maintaining. That alone will fatigue you. My point is NOT to dissuade you from going, but just to make you aware of the potential issues. There are things you can do that will help.
Probably the biggest thing you can do is just stop and get OFF the bike every 100 miles or so whether you need to or not. Just spend at least five minutes walking around, maybe do a few stretches or squats. This does wonders not only for keeping you from getting sore from sitting in one position all day, but it also helps a great deal with keeping you mentally alert. I usually make it a gas/snack stop regardless of whether I need gas.
If you are fine with better living through chemistry, then I suggest the following. The night before you leave, take a full dose of something like ibuprofen before going to bed. Do it again with breakfast. Then do a half dose with lunch and dinner (and maybe before bed if you are sore). Do this each day. It will make a big difference unless you are just one of those young guys that still has good joints
Good gear is critical. If it is cold, you need gear that will keep you warm, not just alive. Cold hands get VERY stiff and if needed in an emergency are SLOW to respond to your command inputs. Keep your torso warm and that will help the rest of your body stay warm. Gear that will keep out the wind and any rain is what you want. A good balaclava helps as well,
Amazon.com: Klim 3116-001-000-000 Balaclava: Sports & Outdoors
This will keep the wind off your neck and face. It is amazing how well this one works. I have it and love it. It also doubles as a goofy looking cap to keep my balding head warm just by pulling it up over my face but leaving it on the top of my head and ears.
While riding, I would avoid big meals. Big meals can make you drowsy, which is rarely a good thing on the bike. Maybe get in a big breakfast if you like so long as you wait a while before setting off on the bikes. Once riding, I like to keep the meals light and maybe snack if I get hungry. Then enjoy the big dinner once off the bike.
You may already use them, but I also highly recommend ear plugs. I just use the cheap foam ones you can get at Wal-Mart, Walgreens, etc.... I take a bunch with me so I don't have to worry about losing them, dropping them, or getting them dirty. The low frequency buffeting from then wind, even in a full face helmet, can significantly contribute to your sense of fatigue over the course of the day. You might not realize it, but it does.
Keep in mind that this time of year, the days are short. So if you plan to do any 500 mile days, you WILL be riding in the dark for a good portion of those days. A GOOD headlight is a must. This time of year is prime time for critters of all sizes to be out and about on the roads in the evenings and at night. If your headlight is not a good one, I would give serious consideration to adding some kind of after market lighting. At a minimum, I'd see if the light housing can handle a bulb upgrade (some will melt with the higher wattage bulbs).
Regardless of where you go, you will likely have to do a long day there and a long day back. The Big Bend area is nice and will likely be warmer than Arkansas. This time of year, if you head to New Mexico and get into any serious elevation (5000'+) it WILL be cold. If you can do it, I'd watch the weather and try to go right after a front passes through. While it might be cold, it usually means clearer sunny days. If you can go during the week instead of on a weekend, even better because the roads will be less crowded and in some places the hotels will be cheaper as well.
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