McLaren have struggled since re-joining forces with engine partners Honda in 2015, finishing ninth in that year's constructors' championship and sixth last year - and Alonso admits it will be hard to get back to the front quickly.
"We still think the start of the season will be a challenge - we can't ignore the fact that we're still coming from a significant step behind the current front-runners," the Spaniard said at Friday's launch of McLaren's new MCL32.
"But I'd like to think we can target the second half of the year as a time when we'll really be able to start making useful performance steps.
"The aim is to look respectable this year - and I'd like to think we can achieve that."
Alonso insisted progress is being made, however, adding: "I think there's a lot to admire about what we've achieved over the past 12 months. Sure, the results haven't really showcased it, but we've really made progress, and I think the whole team has gelled together through those difficult times.
"Now, at a moment of significant regulatory change, we need to capitalise on that period of uncertainty to push forwards – and I'm confident that we've got the expertise to do just that."
Jonathan Neale, McLaren's Chief Operating Officer, was also keen to temper heightened expectations for the start of 2017.
"The journey ahead isn't going to be easy, and I've emphasised that to everyone," Neale said. "We've made progress in the past 12 months, but we're not where we need to be and we expect on-track competition to be fierce.
"Do I believe we'll be back at the front this year? Realistically, probably not quite yet, no. But do I think we'll continue to make meaningful improvement as a team? Absolutely.
"And that's our aim: to make progress by establishing the proper and correct, if sometimes difficult, changes that are needed to go forward.
"We can't predict where that will leave us - particularly on the eve of a new season of regulatory upheaval and uncertainty - but as a team we have many talented and driven individuals and we're restless about continuing to do whatever is needed to make us competitive."