Let me give you a little food for thought... If something truly will add pleasure for your riding, don't hesitate to upgrade.

Before you upgrade, you should realize that the bike isn't what will make you keep up or fall behind. A rider from the Tour de France would take your bike, and ride faster than the fastest rider in the group you want to join on the best bike in the group... A better bike will make a slight difference in your ability to keep up, it won't make a world of a difference, as long as your bike is well maintained.

I am taking a nice 1986 Schwinn and updating everything, but staying with 27" wheels because the brakes won't allow the reach for 700c. The bike will end up costing as much as a new bike, and weigh more, so it won't be worth as much as I will be spending to most people, but it is worth it to me.

So for cost/benefit, you need to be somewhat cautious about whether updating a vintage bike is worth it financially. If what you are looking for is a modern road bike, you will get more modern road bike for $700 by buying an entry level road bike than upgrading a vintage bike. However, for $700, you can take a vintage bike and get something that nobody else has. ;-)

Whatever your decision, just have a goal of enjoying riding the bike! It is never too late to try taking a different direction in your cycling adventure, and learning your specific preferences along the way.