I suppose this has been foreshadowed ever since the first 'safe delivery' charm was accidentally acquired and gifted, to be honest. I do agree they shouldn't be rushing into having this child, but... at the same time, I do get their feelings. I don't think either of them actually cared that much about track and field- it's just what everyone else around them wanted of them. And, for the longest time, it was all Suzuka had... but, now, just like Yamato, she has something important to her, something more meaningful than athletics.
And, with this being a romance story, it's no surprise. It's about the two of them being together. The athletics just ended up being a long-term detour in their lives. This situation forced them to make the choice between continuing to follow that athletic career (which would have ended relatively early, most likely, anyway, after they 'passed their prime' ...) or seeking their own happiness between them, away from the athletics that only ultimately gave them hardship, pressure, frustration, pain... Sure, when they succeeded, there were accolades, but... it didn't really seem like they were actually enjoying themselves, ultimately. They found more fulfillment with each other.
Yeah, it's sappy as hell, but, again... it's a romance. And they're determined to make it work, even if it's earlier than they would have liked. I can appreciate that, to be honest.