Working out travel time from gdistance's conductance values in R?

Introduction

This question is indirectly related to a previous post of mine (Calculating accumulated walking pace from starting location outwards using R?), but asks guidance about a different aspect, that's why I am creating a new post.

I have put togheter some doce (below) to provide a reproducible example.

Aim

Calculating cumulative cost-surface expressing walking time from a given location outwards.

Background

I have followed the code provided by this source (https://mran.microsoft.com/snapshot/2014-11-17/web/packages/gdistance/vignettes/gdistance.pdf) to calculate an accumulated cost surface from a location outwards, using the 'gdistance' package. The cost surface has to represent walking time according to the Tobler's hiking function (https://en.wikipedia.org/wiki/Tobler%27s_hiking_function).

## EXAMPLE ##

library(gdistance)

# create the data to work with
dtm <- raster(system.file("external/maungawhau.grd", package="gdistance"))
A <- c(2667670,6479000)

# define the Tobler's function, re-expressed not in Kmh but in meters per hour
tobler.mperh <- function(x){ (6 * exp(-3.5 * abs(x + 0.05))) * 1000 }

altDiff <- function(x){x - x}
hd <- transition(dtm, altDiff, 8, symm=FALSE)
slope <- geoCorrection(hd,scl=FALSE)

speed <- slope
speed[adj] <- tobler.mperh(slope[adj]) #this should be speed in m/h

#sanity check for speed:
plot(raster(speed)) #flat areas should feature around 5000 m/h

#geocorrection divides speed by distance (i.e., cells' centers)
conduct <- geoCorrection(speed, scl=FALSE) # conductance = speed/distance = 1/travel time
plot(raster(conduct))

cost_surface <- accCost(conduct, A)
plot(cost_surface)      # = 1/travel time

Workflow followed

The abovementioned PDF was implementing Tobler's function not to create an accumulated cost surface but to calculate shortest path. With reference to the code above, I am rather confused about how to work out travel time from the last result of the code, i.e. the accumulated cost surface.

In other words:

all seems fine till the calculation of the speed (meters per hour) data (see code flagged by #sanity check for speed). The speed raster I got (see below) seems to make sense, since flat areas feature a speed of about 5000 m/h (i.e., 5 Kmh) as predicted by the Tobler's function. The above is confirmed by the following chart of speed vs slope: The subsequent operation that uses geocorrection divide the speed (m/h) by the distance (i.e., distance between cell's center), so producing what is called conductance. The latter (according to the abovementioned PDF) is equal to 1/travel time.

The accumulated conductance looks like the image below (see plot(cost_surface) # = 1/travel time): Issue

In an attempt to devise the travel time (tv), I simple calculated the reciprocal of the accumulated cost, the latter being based on the conductance data. But taking the inverse produces a raster that seems to make no sense:

tv <-cost_surface
values(tv) <- 1/values(cost_surface) Question:

Is the issue caused by including 0 values into the calculation of the reciprocal?