DD and I did transitional preschool together when she was 2. Started out as a Mom & Tot class, then as the year progressed, she started to go on her own until she was going 1 1/2 hours by herself once per week.
This year she is in 3 yo preschool, M-W-F for 2 1/2 hours. We are sending her to camp this summer for the interaction, and really it will just feel like a continuation of school since it is at her current preschool with the same teachers.
Next year she will be in a "Super 5s" class with other older 4s and 5 yos, 4 days a week for 2 1/2 hrs.