Well, if you rush straight to the end of the map to reach Isabel, then of course there's nothing unrealistic about it, but it's not interesting to me. What's the point of playing the campaign if you're just trying to rush from one scenario to the next? Of course, it's a matter of taste; some people probably enjoy speedruns, but I like to explore the entire map, level up my hero as much as possible, and, of course, give the annoying AI a good beating.
The strangeness and illogicality of Heroes 5 is that the slower you play, the fewer final points you get for the campaign/scenario. But it's nice to see yourself in your own top 10 of the game.