Correct play or unnecessary gamble?

[Gus]

online, one table tournament, 10$+1. All player unknown to me. The small blind (leader in chips at about T5000) seems to be on the loose side.

Blinds 50/100, 7 players left, I'm on the button with about T2500 (basically 2nd so far... another player has about the same as me).

I'm dealt A [img]/forums/images/icons/diamond.gif[/img] 7 [img]/forums/images/icons/diamond.gif[/img] . Folded to me I make it 250 to go, the small blind raise to 400, big blind folds, I call (pot 900).

Flop : 4 [img]/forums/images/icons/diamond.gif[/img] 5 [img]/forums/images/icons/spade.gif[/img] 6 [img]/forums/images/icons/club.gif[/img]
giving me an open ended straight draw. SB checks, I bet the pot, he calls.

Turn : 9 [img]/forums/images/icons/diamond.gif[/img]
sb bets 1000... That puts me all in if I call... I call, don't improve on the river and lose to AKo.

After this hand I felt I had been gambling on every street, whereas I should have waited for a bit of a stronger hand before having a go at the chip leader.

Any comment welcome please.