Although the Red Bull Technology car that Toro Rosso will use has shown promise in early testing, Tost thinks the limited running it has had with its own version means it has to play catch-up.
Plus, with star driver Sebastian Vettel having switched over to Red Bull Racing, changes in the team will take some time to settle down.
"It will be quite difficult to repeat last year's success because to be successful in F1 you need continuity - too many changes are toxic," Tost told the office Formula 1 website.
"Sebastian Vettel has left us for Red Bull Racing and it will not be easy to replace him as he showed a fantastic performance. Nevertheless, we will do our best and I am convinced that with this car we will do a proper job.
"But as became obvious from the times at the last all-team test, the teams are very, very close and we have to push very hard to stay in the midfield. Anything that comes on top will be very welcome, but midfield is what we are aiming at."
With the team having only tested once with its new car alongside its rivals, Tost is unsure of where his team stands performance-wise at the moment.
The issue has been further clouded by the time the team lost with a suspension problem during that sole test at Barcelona.
"The car looks fantastic," explained Tost. "Red Bull Technology and Adrian Newey have done a great job. After only one test it is difficult to say where we stand performance wise - this we still have to find out.
"Overall last week's running was satisfying, even though we had a problem with the front suspension on day one, with only 20 laps of running as we decided to stop for safety reasons.
"After investigating the problem we were able to give the green light for the rest of the test and we faced no further issue."