I think that if she is doing 4.5hours normally, then after a 3 hour nap to stay awake for 5 hours is to be expected!! Is it a good sign....well that depends on what you think of long A times and one nap!!! LOL - for me I hated it, I'd rather my DS still be on 3.5h awake and taking 2 naps, I find that 5 hours of him being awake is too much for ME to take! He's so full of energy and so spirited and demanding that I need him to get out of my hair after a few hours and more than just once per day!!
I don't really think you can judge if it is a good/bad thing but it is probably normal. I found that when we were on 2 naps A times were predictable but when we went to one nap A times were NOT so predictable...meaning that just because she does x amount of A time after x length of nap right NOW does not mean that is always it....there is only one chance to rest in the day and so during periods of catch-up (for instance) a 3 hour nap might mean she is still tired enough to go to bed at 4h or 4.5 h.