The romance in
Nodame Cantabile extends over an initial season of 23 episodes and two twelve-episode sequels. It's pretty one-sided at the beginning, with Nodame having more of a "crush" on Chiaki and his being unresponsive. (Chiaki is the very model of a male "tsundere" character.)
Nodame Cantabile has everything you asked for, and more.
Another possibility is
Saiunkoku Monogatari. The heroine is a feisty young woman who is brought into the imperial household as a concubine because of fears that the naive young emperor might be a homosexual. Their relationship becomes much more complex as the show unfolds and extends over two, 39-episode seasons. This show is hard to find nowadays as it has fallen into licensing limbo, but a little Googling might give you some clues.
A third option might be
Planetes, a story about space "trash" collectors. Like
Nodame, it's a relationship between a young woman and her slightly older senpai.
Planetes has a variety of parallel themes; the romance is just one part.