Falkirk and St Mirren have strengthened their squads over the summer while Motherwell have been lifted by the arrival of new manager Mark McGhee and the return to fitness of a number of key players.
Killie have finished fifth in the SPL for two years in succession and will be looking to at least equal that feat with Lilley and the likes of David Fernandez and Eric Skora all returning from long lay-offs.
But Lilley believes it will be harder to achieve given the improvement of other teams.
On Monday Killie host a Dundee United side fresh from a 1-0 opening win over Aberdeen that built on their impressive form for much of last season after Craig Levein took over in October.
Lilley, 29, said: "I think with the way United started and the way they looked in pre-season they would be hoping to be top six as well.
"But it's a hard league. Outside the Old Firm there is not a great deal between the rest of the teams.
"We are trying to aim for top six again this season, which would be a great achievement for us, three years in a row.
"A lot of it is down to budgets. There is not a lot of money for managers to spend, so that's why there is not much between the teams."