How to predict ETA using Regression?
I suggest to calculate the Haversine distance between two points, and fit a linear regression to find the relation between the Haversine distance and the trip duration.
So your regression will be
$duration_t = timestamp_t - timestamp_{t-1} = \alpha + \beta*d(point_t,point_{t-1})$
Where $d$ is the Haversine distance. $point_t$ is a lat/long pair at time $t$.
Note however that there's an assumption that the user drives at the same speed. If half of your data was gathered while walking and half while driving, then your relation between time and ETA is possibly not linear.