Here is one way of doing it:

With MyCTE as(

select RT1.*, RT2.TimeStamp as EndTimeStamp,datediff(second,RT1.TimeStamp,RT2.TimeStamp) as DiffInSeconds

from RunningTime RT1 LEFT JOIN RunningTime RT2 ON RT1.Id = RT2.Id - 1)

select *,

right ('000' + CAST (DiffInSeconds / 60 /60 as varchar(4)), 3) -- Hours part

+ ':' + right ('0' + CAST (DiffInSeconds / 60 % 60 as varchar(2)),2) --Minutes part

+ ':' + right ('0' + CAST (DiffInSeconds % 60 as varchar(2)),2) --seconds part

from MyCTE

Adi