If you are asking if that is too early to ride the Southern tier , I'd say no. I did it from Mid February to Mid March (I only went from San Diego to Pensacola) and found that a nice time to be on the route. I hate hot weater and don't mind chilly nights. I saw a little snow on the roadside on the tops of a couple passes, but the roads were always clear. There was a chance for needing to wait out a few days while snow melted, bit that didn't happen. It was really cold one night (teens F) and there were lots of nights with frost, but I found it pleasant enough and I think it probaly hit at least 50 F every day. A few weeks earlier might be colder and more chance of snow so you may need to wait a few days for some snow melting.
If you are asking about timing for the loop... Timing is critical for a trip like that and with no experience you can't really know what your pace will be. Timing will be most critical will be in the mountains in the Northwest corner of your loop around the country. Winter hits early and stays late there, so you need to arrive at a suitable time not before June. With not much idea of your pace that will be easy to get wrong.
Starting in the SW you will be doing the most boring half of a loop around the country first. There were interesting people and good food in the south. There were even a few spots with good scenery, but there were also endless stretches of monotonous brown nothingness with long stretches between services. This may be discouraging to a new tourist on what will probably be a 6 month ride. Still that is the most likely way to hit all parts at the optimum time of year.
You'd have a better chance of successfully staying motivated if you stated out doing a more inspiring section or just did a long shake down ride first. I'd suggest doing a more interesting scenic trip with more services like the Trans America or the Pacific Coast before you do this, but you'd want to do them in season. Personally I found the ST okay for something to do in the winter, but don't really recomend it for someone starting out unless they will enjoy day after day of brown empty sage brush (yes some people will love west Texas).
If you are really set on doing the loop first, starting in the SW corner and going CCW is the way to go. The key is picking the right start time for your pace. The problem is that you will be starting maybe 8K miles (more depending on your route) from the mountais that you need to hit at the right time. You'd need to check your actual route and get a real number, 8K is a guess. Folks might reasonably average 30, 50, or 80 miles per day counting rest days. The range could be even wider, but that is a reasonable range. That would mean it might take 267, 160, or 100 days respectively. You probably don't want to be in the mountains before June at the earliest or it might still be snowed in or even have winter conditions. You need to calculate your planned mileage, number of says until safe riding in the mountains, and likely average daily mileage to pick a start date. That said I don't know how someone who hasn't ridden in 45 years will have any idea what their average daily mileage will be. The good news is that you can get through the mountains any time through the Summer.
I still think riding the Pacific Coast, Trans America, or maybe just a shorter warm up like the Oregon coast first would be a great idea.
