The match ended up being quite short (two minutes and 48 seconds), in fact the entrances and pre-match build were quite a bit longer than the match. That was by design and always the plan, according to this week's Wrestling Observer Newsletter.
Part of the reason that Undertaker never appeared on WWE programming in the build to the match was to somewhat downplay fan expectations heading into the match. While WWE had always planned on the bout happening and wanted to strongly give fans the impression that it would be happening at WrestleMania, they didn't want fans to head into the show with the expectation that it would be an all time classic match given twenty plus minutes like Undertaker's past WrestleMania contests.
The health situation of The Undertaker was certainly the reasoning for this, and given his inability to perform to his usual capability at last year's show against Roman Reigns, the company wanted to keep his time in the ring brief. In the end, that was the best decision and the segment ended up being an overall success. In fact, in light of how things went with Reigns and Brock Lesnar, you could make the argument it could have closed the show, even with the limited amount of ring time.
We're not sure how much more we will be seeing The Undertaker in a WWE ring moving forward, but the fact that he will be wrestling Chris Jericho in a casket match at Greatest Royal Rumble in two weeks is certainly a good sign. If he was ready to retire, it's highly unlikely he would be working a match like this on such short notice.
The real interesting part of the match with Jericho will be to see what kind of length the bout is given. After getting that first match back out of the way with Cena, WWE could be willing to give him some more time against Jericho. The amount of time the match is given will certainly be a clue to how often we will be seeing Undertaker performing in a WWE ring in 2018 and beyond.