He wasn't. There's zero lore sources mentioning that. The only lore source saying anything him being a second-in-command (other than being one to Garrosh during WotLK) is him being made one by Orgrim after he took over the Horde. I.e. after the First War was already over. And with that in mind whether Trolls were a problem prior to Zul'jin or not matters nothing. If you base Saurfang having more experience than Sylvanas due to him being second-in-command of the Horde being more important than the war against Trolls prior to Zul'jin, and only Trolls post Zul'jin counting, then you end up with Saurfang still being on the losing side in terms of time spent gaining that experience.
And yet we know absolutely nothing about his involvement in Third War other than vague mentions about him being a veteran of it. He's a veteran of the war against the Iron Horde and Fel Iron Horde too. All he did during that time was visit the Garrison every now and then. And we know nothing about how many battlers were fought precisely during these wars or most other wars, let alone which were led by Saurfang.
Which still ends with Sylvanas most likely being much older and as such having done such things longer than Saurfang.
Alleria went with one small group to pursue her vendetta. Sylvanas led the whole army of Quel'thalas.
There wasn't anything to hold, Alliance and the Horde weren't at war then. He just led a mixed forces army. Just like Sylvanas did at Broken Shore and will do in Legion.