Seoul is huge. Individual neighborhoods aren't that large, and it's not too difficult to explore a neighborhood in an hour or so. But there are tons of little neighborhoods, and Seoul's important sights are spread all over the city and metro area.
Getting around takes time, especially going from one end of the city to the other, but the subway system is quite dense and inexpensive, and the trains are quite frequent. Tokyo as a tourist destination is smaller, though the Tokyo metro area is larger.
Tokyo has unbelievable accessibility, but the network is a combination of JR, private rail, and Tokyo Metro lines. This means that the fastest, easiest, or most convenient route might involve changing companies, drastically increasing the overall fare. Seoul's network functions under one organization with one fare structure, which helps keeps rates low even when transferring lines.
As I mentioned, the lines are always busy, and during rush hour the cars are packed. People don't push and shove onto the trains like they do in Tokyo, though. There's a tap-and-go card that works on the subway, on buses and taxis, and in stores, which I highly recommend. It provides a discount (especially useful when using the subway and buses) and is much more convenient than buying a token for every trip and getting a refund for that token when you leave. If you're going to stay in Seoul for more than a couple days it pays for itself.
But don't let that deter you! Seoul is a wonderful place to visit with great food, sights, shopping, and transportation!